1. 推動醫療曝露品質保證作業研討會 台灣地區遠隔治療及192Ir近接治療劑量標準之校正追溯 李 振 弘
李 振 弘
國家游離輻射標準實驗室
原子能委員會 核能研究所

2. Contents
一、游離輻射標準現況說明
二、192Ir近接治療劑量標準
三、校正服務
四、遠隔治療劑量標準

3. 中華民國實驗室認証(CNLA)認可的實驗室
一、游離輻射標準現況說明
台灣地區游離輻射領域追溯體系
國際度量衡局(BIPM) 追溯 追溯 比對
國家游離輻射標準實驗室(NRSL)
其他國家的標準實驗室(NMIs) 追溯 追溯
(醫療曝露)
中華民國實驗室認証(CNLA)認可的實驗室 追溯
儀器或射源使用者

4. 1993-2004建立之游離輻射標準及應用領域 應用領域 量測標準 放射診斷 放射治療 核子醫學 實驗室認證 相互認可協定 國際趨勢
10~50 kV X射線空氣克馬 V
100~250 kV X射線空氣克馬
137Cs 加馬射線空氣克馬
60Co 加馬射線空氣克馬
60Co 加馬射線水吸收劑量
192Ir 參考空氣克馬
90Sr/90Y 組織吸收劑量
252Cf 周圍/個人等效劑量
241Am-Be 周圍/個人等效劑量
4πβ-γ 符合計測(活度)
4πγ 游離腔(活度)
2πα/β 比例計數器(發射率)

5. 已建立 光子標準 高能量 中能量 低能量 發展中 待建立 工業用高劑量標準(化學劑量計) TG21、TG51 線性加速器
15 MeV
高能量 中能量 低能量
工業用高劑量標準(化學劑量計)
(95-97)
TG21、TG51
線性加速器
線性加速器(熱卡計)
(98以後)
2 MeV
60Co Co
空氣克馬 水吸收劑量
1.25 MeV
192Ir參考空氣克馬(近接治療)
(95-97)
137Cs空氣克馬
662 keV
kV空氣克馬(ISO/narrow)
(94-95)
192Ir參考空氣克馬(近接治療)追溯PTB
300 keV
kV空氣克馬(ISO/low)
(95-96)
kV空氣克馬(BIPM)
125I, 103Pd參考空氣克馬(近接治療)
200 keV
10-50 kV空氣克馬(BIPM)
電腦斷層掃瞄(CT)X光劑量
(96)
100 keV
23-35 kV空氣克馬(乳房攝影)追溯NIST
血管X光攝影劑量
(97)
50 keV
10-50 kV空氣克馬(BIPM)
數位式及胸部X光劑量
(98)
10 keV

6. External radiotherapy
Brachytherapy
近接治療
External radiotherapy χ
遠隔治療
Radiotherapy

7. 二、192Ir近接治療劑量標準
IAEA-TECDOC-1274：Calibration of photon and beta ray sources used in brachytherapy
Guidelines on standardized procedures at Secondary Standards Dosimetry Labotatories (SSDLs) and hospitals (March 2002)

8. 1.Characterization of brachytherapy source
Gamma ray sources
The recommended quantity for the specification of the gamma sources is the reference air kerma rate, defined by the ICRU report 38, 58, 65 as the kerma rate to air, in air, at a reference distance of one meter, corrected for air attenuation and scattering.
Beta ray sources
The recommended quantity for specification of beta ray sources is the reference absorbed dose rate in water at a reference distance from the source. The reference distance differs from one type of source to another.

9. 2.Specification of brachytherapy sources used in Taiwan
ICRU 38 notes that conventional dose rate where the prescribed dose at the point of dose prescription
“Low” dose rate (LDR) : 0.40 2.0 Gy h-1
“High” dose rate (HDR)  12.0 Gy h-1

10. 3. High dose rate 192Ir Source calibration at PDSL
Primary standards for HDR 192Ir sources are not available.
PTB/Germany comprises the evaluation of the entire calibration function of the ionization chamber between 30 keV and 60Co radiation, and a subsequent interpolation for the 192Ir emission lines weighted with their emission probability.
 The overall relative uncertainty is 2.5% (k=2).

11. free air chambers for low and medium energy x-ray qualities
To obtain the response functions, the chambers were calibrated against primary air kerma standards:
free air chambers for low and medium energy x-ray qualities
graphite cavity chambers for gamma rays for 137Cs and 60Co
ISO narrow-spectrum x-ray qualities used for chamber calibration
Free air chamber at INER
Graphite cavity chamber at INER

12. The set-up for the air kerma calibration for HDR 192Ir sources at 1 m
(1)Without a shadow-shield：primary + room scattered + environment radiation
(2)With a shadow-shield：only room scattered + environment radiation
(1)-(2), corrected for attenuation in air  air kerma rate in vacuum which is identical to the reference air kerma rate in the case of measurements at 1 m dtstance

13. 4.HDR 192Ir Source calibration at SSDLs and hospitals
4.1 Free in-air measurement
The reference air kerma rate, KR, may be determined from measurements made free in-air using the equation:
KR=NK·(Mu/t)·kair·kscatt·kn·(d/dref)2
NK： the air kerma calibration factor of the ionization chamber
Mu： the measured charge collected during the time t and corrected for ambient
temperature and pressure, recombination losses and transit effects during
source transfer in the case of afterloading systems
kair ： the correction for attenuation of the primary photons by the air between
the source and the chamber
kscatt： the correction for scattered radiation from the walls, floor, measurement
set-up, air, etc.
kn ： the non-uniformity correction factor, accounting for the non-uniform
electron fluence within the air cavity
d ： the measurement distance
dref ： the reference distance of 1 m.

14. 4.1 Free in-air measurement (con’t)
4.1.1 Ionization chambers to be used
For HDR sources, ionization chambers with volumes greater than 0.5 cm3 can be used (e.g. Farmer 0.6 cm3 chamber). For 192Ir calibrations, it is recommended to use chambers that have a variation of the air kerma calibration factor of less than 5% between 60Co and 60 keV.
4.1.2 Air kerma calibration of ionization chamber (NK)
The air kerma weighted average energy of an 192Ir brachytherapy source is 397 keV, the principle proposed by Goetsch is to calibrate the chamber at 250 kV x-ray quality and at 137Cs, or at 60Co if a 137Cs beam is not available. A typical X ray beam that can be used for calibration at the SSDLs is 250 kV, added filtration of 1.0 mm Al and 1.65 mm Cu, and a HVL of 2.50 mm Cu.

15. 4.1.2 Air kerma calibration of ionization chamber (NK) (con’t)
137Cs calibration point
The ionization chamber wall (inner wall and cap) must be thick enough (0.36 g/cm2 ) to block all electrons emanating from the source or capsule, and to provide CPE in the 137Cs beam.
Air kerma calibration factors, NK, for both the 137Cs and x-ray beam must be determined with the build up cap (equivalent wall) in place for both beams. The factor Aw for the attenuation of the cap and scattering effects of the chamber wall must be taken into account.
where x = (t/ 9.3×1022) for a wall thickness of t electrons/cm2

16. 4.1.2 Air kerma calibration of ionization chamber (NK) (con’t)
60Co calibration point
In the event that there is no 137Cs beam energy at the SSDL, a 60Co beam may be used as the high energy point using the appropriate build up cap and wall thickness for 60Co, 0.5g/cm2. The weighted interpolation factors are given by the following equations
and
where and are the air kerma weighted average energies of 192Ir and 60Co gamma rays, respectively, and represents the effective energy (131 keV) of the 250 kV x-ray beam. This results in the following equation for NK,Ir with the weighted air kerma values

17. Table IV includes Aw factors for different ionization chambers
Table IV includes Aw factors for different ionization chambers. If the chamber in use is not listed in the table, then Aw can be set to 1.000

18. 4.1.3 Correction factors for free in-air measurements
Measurement distances
A practical criteria is that the distance between the chamber center and the center of the source must be at least 10 times the length of the source in order to ensure that the error introduced due to the point source approximation is less than 0.1%. For HDR source calibrations, the measurement distances can be selected around the optimum distance (e.g. between 10 cm and 40 cm).

19. 4.1.3 Correction factors for free in-air measurements
The scatter correction factor
Two methods have been used to determine the scatter correction: the multiple distance method and the shadow shield method
In the former method, the air kerma rate due to scattered radiation is assumed to be constant over the measurement distances.
It is essential in this method that the changes in distance be precise and accurate, in order to derive the correction c that yields the “true” center-to-center source to chamber distances, d'.
d' = d + c
where
d ： the apparent center-to-center source chamber distance

20. Where M'd is the reading from primary photon only.
The charge readings after application of the corrections discussed above are denoted here as Md referring to the nominal distances d, A constant reading Ms due to scattering is included in each reading：
Md = M'd +Ms
Where M'd is the reading from primary photon only.
f = M'd (d')2 = (M d -M s)(d+c) 2= constant
Any group three such equations, for three distance, can be solved for three unknowns Ms, c, and f. Making measurements at more than three distances overdetermines that result, and averaging of several solutions to minimize the error.
f = M'd (d')2 = (M d -M s)(d+c)
(d’)2 dK/dt = (NK) f / dt

21. The shadow shield method has mainly been used to determine the scatter correction factor at a distance of 1 m., a cone of a high Z material is placed between the source and the chamber in order to prevent the primary photons from reaching the chamber. The ratio of the measured charge with and without the shield in place can be used to calculate the scatter correction factor.

22. Table V shows the results of a few experimental determinations of the scatter correction using the shadow shield method. In 192Ir dosimetry it has been shown that the scatter correction factors obtained with the two methods are in a good agreement.

23. 4.1.3 Correction factors for free in-air measurements
The non-uniformity correction factor
In the measurements of brachytherapy sources free in-air, the non-collimated geometry, with high divergence of the incident photons, Due to the non-uniform photon fluence over the wall, the generation of electrons from the wall varies significantly from place to place in the wall. The net result of this is a non-uniform electron fluence in the air cavity of the chamber. The non-uniformity correction factor, kn depends on the
shape and dimensions of the ionization chamber (spherical,
cylindrical, internal radius and length);
measurement distance and the source geometry (‘point source’,
line source, etc.);
¾material in the inner wall of the chamber;
¾energy of the photons emitted from the source.

24. The relationship between the two theories is given by
Kn=1/Apn(d)
is the non-uniformity correction factor obtained from the isotropic theory and is the non-uniformity correction factor according to the anisotropic theory. A'pn(d) takes into account the anisotropic electron fluence within the air cavity and the degree of anisotropy is given by the energy and material dependent factor w.
Table VI the w values for some commonly used inner wall materials.

27. For spherical ionization chambers, w = 0, and the non-uniformity correction factors given by isotropic theory can be directly applied. The Apn(d ) factors for spherical chambers are reproduced in Table X.

28. 4.1.3 Correction factors for free in-air measurements
Correction for the attenuation of primary photon in air
For determination of the reference air kerma rate from the measured air kerma at the distance d, it is necessary to correct for the attenuation of the primary photons between the source and the ionization chamber. Table XI gives the kair correction factors at different distances between the source and the ionization chamber.

29. 4.1.3 Correction factors for free in-air measurements
Correction for transit effects, leakage current and
recombination loss
While the source moves into the measurement position, and then away after the measurement, the detector measures a signal, referred to as the transit signal. This transit signal strongly depends on the source-to-detector distance, and is significant at the distances used in calibration.
Several techniques can be used to eliminate the transit component of the signal:
Using an externally-triggered electrometer to collect charge during an
interval after the source has stopped moving
¾ Subtracting two readings taken for differing intervals to eliminate
the transit charge common to both.
Using a current reading after the source has stopped moving (if the
signal is large enough).

30. The importance of electrical leakage currents in the individual dosimetry system should be evaluated since the signal levels measured during calibration are typically 50 to 100 times less than usually encountered in teletherapy measurements. This can be significant for most thimble or Farmer type ionization chambers. Generally if the leakage is greater than 0.1% of the signal, it should be taken into account.
A correction is also needed for the recombination losses and for the ambient temperature and Pressure.
Final  The reference air kerma rate, KR, may be determined from measurements made free in-air using the equation:
KR=NK·(Mu/t)·kair·kscatt·kn·(d/dref)2

31. 4.2 Calibration using well type ionization chamber
Using well-type chambers, the reference air kerma rate can be calculated generally using thr following expression
KR=NK kp Imax kion
where
NK：the air kerma rate calibration factor(Gy h-1 m2 A-1) taken from the
calibration certificate
kp ：the correction factor for the difference between the actual air pressure
and temperature and the reference chamber calibration conditions
Imax：the maximum measured ionization current value with the well type
chamber (including the electrometer calibration factor).
kion：the reciprocal of the ion collection efficiency factor Aion calculated as
follows
Q1 and Q2 are the charge readings at nominal (300 V) and half (150 V)
potential

32. NK=KR /( kp Imax kion )
4.3 Calculation of well type ionization chamber calibration factor at INER
NK=KR /( kp Imax kion )
Mallinckrodt Medical B.V. D35A0832 HDR 192Ir
KR：47.9 mGy m2 h-1 ( :00追溯至德國PTB, 不確定度2.5%, k=2)原廠測試報告48.85 mGy m2 h-1 ( :00), 差異2%
： ×10-3 d-1
Remote afterloader and well type chamber at INER

33. Nucletron afterloader 對PTW HDR well-type chamber Imax設定位置為830 mm

34. 量測不確定度3% (k=2)

35. 4.4 Calibration using well type ionization chamber
Quality control of well type chamber measurement
The complicated energy spectrum of HDR 192Ir includes about 40 energies falling approximately between 50 keV and 700 keV and with an average energy of 397 keV. In practice, it is possible to use 241Am (average energy 60 keV) and 137Cs (average energy 661 keV) check sources for this purpose. The stability of the output of a well type chamber should be check at least 4 times per year. If the periodic constancy checks remain the same to within 1 %, it can be assumed that the calibration factor for response of the chamber does not change significantly with time at these energies, it may be concluded that the chamber’s calibration factor for HDR 192Ir sources has not changed. Further, if chambers is used for 192Ir source calibrations, the re-calibration interval should be shortened to 2 years.

36. 4.5 HDR 192Ir brachytherapy source traceability Chain （in Taiwan）
Primary Laboratory
Well type Chamber
(Primary Lab.)
Standard Chamber
Standard Source
User
Well type Chamber
(user)
User’s Source KR .

37. 三、校正服務 國家游離輻射標準實驗室網站新功能 儀器校正進度查詢 或
點選中文/校正服務/儀器校正進度查詢 或 或

38. 送校方式與校正時程 送校件 送校方式 校正時程 備註 遠隔治療劑量校正追溯 Cavity chamber 客戶自送或 委託廠商送達
按現行作業流程
已開放校正服務
192Ir近接治療劑量校正追溯
Well-type chamber
(優先度1)
客戶自送委託廠商送校
(1)兩週前預約，3工作天完成
(2)無預約，接收日起7工作天完成
(3)校正完成後5工作天內寄發校正報告
(1)94.7.1開放校正服務
(2)如有急需可先傳真報告
(3)提供現場游校
192 Ir近接治療射源
(1)廠商送校
(2)現場游校
(1)兩週前預約，當日完成
(2)校正完成後3工作天寄發校正報告

39. 游離腔及192 Ir射源校正費用 加馬射線空氣克馬：基本費新台幣八千元/部 (能量範圍137Cs、60Co，每增加一能量點加新台幣二千元)
X射線空氣克馬：基本費新臺幣八千元/部（能量範圍30
~250 kV，每增加一能量點加新臺幣二千元）
60Co水吸收劑量：新臺幣八千元/部
Well-type chamber：預估新臺幣一萬五千元/部
192Ir射源：預估新臺幣一萬元/個

40. 提昇實驗室人力運用與客戶送校儀器之校正效率
校正季規劃
提昇實驗室人力運用與客戶送校儀器之校正效率
92及93年每月校正數量統計

41. 校正季規劃實施方式： 1.每年7-9月開放3個月提供HDR well-type chamber校正。
2.每年7-12月提供cavity chamber校正。
3.研討會歸納客戶意見。
4.以 及書面方式告知。
6.於實驗室對外網站公告。
7.非校正季時，假如仍有客戶送件，
請櫃台告知新措施
收件後校正期程可能延長

42. 四、遠隔治療劑量標準 1. TG-21 protocol (Med Phys, 1983)
TG-21 protocol has two major components:
Part I: How to obtain the calibration factors Nx (exposure) and Ngas(dose to cavity gas) in a Co-60 beam.
if only Nx is provided by the standard laboratory, then the user needs to convert Nx to Ngas.
Part II: How to use Ngas in a user’s beam, which can be any modalities (photon or electron beams) and energies, and the chamber can be placed in a plastic medium.
Dose to cavity gas  dose to medium  dose to water

43. primary standard (3 cm3 、10 cm3 及30 cm3 spherical chambers) and calibration system of 60Co air kerma

44. Comparison of standard of air kerma for 60Co
RNMI, BIPM
60Co air kerma rate measurement uncertainty (0.48 %, k=2) at INER

45. 60Co air kerma calibration factor (NK) uncertainty evaluation
Air kerma calibration factor (NK) uncertainty (0.6 %, k=2)
INER calibration certificate
NK(Gy C-1)及Aion((4-Q1/Q2)/3)
NX = NK/8.79210-3(Gy C-1)
TG-21 NX(R C-1)

46. TG-21 protocol (Part I - 60Co beam)
Ngas：Cavity –gas calibration factor (Gy C-1)
NX：Exposure calibration factor (R C-1) (INER calibration certificate)
k：2.58 × 10-4 (C kg-1 R-1)
W/e：33.97 (J C-1)
Aion：Ion-collection efficiency in the user’s chamber at 60Co exposure
(Calibration certificate)
Awall： Wall correction factor in the user’s chamber at 60Co exposure (Table II or III)
：Faction of ionization due to electrons from chamber wall (Fig. 1)
：Stopping –power ratio(wall/air, cap/air) (Table I)
：Energy-absorption coefficient ratio(air/wall, air/cap) (Table I)

47. TG-21 protocol (Part II - user’s beam)
M：Ion chamber reading in user’s beam (correction for polarity
, electrometer, temperature and pressure)
Pion：Ion-recombination correction factor applicable to the calibration o f user’s beam (Fig. 4)
Prepl：A factor that corrects for replacement of water phantom by an ionization chamber (Fig. 5)
Pwall：Wall correction factor at user’s beam (  from Fig. 7, stopping –power ratio from Fig. 2 or Table IV, energy-absorption coefficient ratio from Table IX)

48. 2. TG-51 protocol (Med Phys 26, 1847-70, 1999 )
The TG-51 protocol is based on “absorbed dose to water” calibration (also in a Co-60 beam)
The chamber calibration factor is denoted
The calibrated chamber can be used in any beam modality (photon or electron beams) and any energy, in water.
The formalism is simpler than the TG-21, but it is applicable in water only.

49. Primary standard (pancake chamber) and calibration system of absorbed dose to water for 60Co
100 cm
5 g/cm2

50. Comparison of standard of absorbed dose to water for 60Co
RNMI, BIPM
Measurement uncertainty (0.54 %, k=2) of absorbed dose rate to water for 60Co at INER

51. Uncertainty evaluation of calibration factor ( )
of absorbed dose to water for 60Co
Absorbed dose to water calibration factor ( ) uncertainty (0.6 %, k=2)
INER calibration certificate
TG (Gy C-1)

52. TG-51 protocol Requires absorbed dose to water calibration factors,
Conceptually easier to understand and simpler to implement
Requires a quality conversion factor, kQ ,change in modality, energy, gradient

53. TG-51 protocol kQ＝Quality conversion factor(kQ=1 for 60Co)
Pion = Ion-recombination correction factor
PTP = Temperature-pressure correction factor
Pelec = electrometer correction factor
Ppol =polarity correction factor of ion chamber
Mraw = uncorrection ion chamber reading

54. 3.Comparison results for TG-21 and TG-51 dosimetry protocols
參與單位：核能研究所國家游離輻射標準實驗室
醫學物理學會
國內13家醫院放射腫瘤科
(北：7, 中：3, 南：2, 東：1)
時間：
成果： accepted by Radiation Measurements SCI Journal

55. The characteristics of farmer-type chambers used in this comparison
Cylindrical chamber waterproof sleeves made in INER
NE-2561(NE-2611，0.3 cm3)
NE-2571 (0.6 cm3)
PTW-30001(0.6 cm3)
 The air gaps between waterproof sleeve and chamber wall<2 mm(AAPM TG-51)
 PMMA material and thickness<1 mm

56. The distribution of /NK calibration coefficients ratios for the cylindrical ionization chambers
ND,w/NK
± 0.006

57. Estimated relative standard uncertainty (%) in the determination of absorbed dose to water at the reference depth in high energy photon beams using the TG-21 NX-Ngas and TG-51 ND,w formalisms (Huq and Andreo, 2001)
TG-21
TG-51
Step 1：User chamber calibration factor
Combined uncertainty
Ngas
0.8
0.6
Step 2：User beam mesurements
Combined uncertainty in dosimeter reading a
0.9
Step 3a：Quantities and perturbation factors for the user beam
Combined uncertainty in stopping power ratios, perturbation factors and their assignment to beam quality
Step 3b：Beam quality correction kQ
1.0
Combined uncertainty in
1.5
1.4
aIncludes long-term stability of the dosimeter, establishment of reference conditions, measurement of beam quality , and dosimeter reading relative to timer or beam monitor

58. Comparison results of absorbed dose to water for 6 MV and 10 MV photon beams determined by following the recommendations of the TG-21 and TG-51 protocols

59. Quality assurance for the switch of photon reference dosimetry between TG-21 and TG-51 protocols

60. Thanks for Your Attention